1. Field of the Invention
The present invention relates to polymer gels, and particularly to a gelling agent for water shut-off in oil and gas wells that enables retarding the fluid flow of water from subterranean oil and gas reservoirs using gel compositions formed by crosslinking water soluble polymers (such as polyvinyl alcohol) with urea-formaldehyde oligomer.
2. Description of the Related Art
Normally, when oil reservoirs become mature, most oil reservoirs produce water mixed with oil in their production lifetime. In some cases, even though substantial flows of hydrocarbons have been shown, water production is so great and water disposal costs so high that hydrocarbon production is not economical. Therefore it is desirable to find a way to reduce or shut off the flow of water while permitting hydrocarbon production to continue.
If the water production is reduced by some treatment (such as polymer gels) for water shut-off, then this might have a positive effect on increasing oil production. The excessive water production causes many problems and becomes very costly when the water cut increases. Water production could be a result of many reasons, including weak formation due to fractures connecting the water zone with the oil-producing zone. It can also be caused by water coning, which is the most common case, due to the high pressure differential between water and oil zones. Other factors that may cause water production include micro-cracks in cement sheet, closeness of perforations to the water zone, and high oil/water viscosity ratio, among other things.
For radial flow, where there are no fractures in the oil or gas well, the polymer gel material could be considered effective if it can reduce the permeability to water by more than a factor of ten and preferably by more than a factor of twenty. At the same time, the gel must reduce permeability to oil by less than a factor of two, if oil zones are not protected during placement. There is no universal treatment method to treat excessive water production in producing oil wells. However, several techniques were developed and used, which include: mechanical isolation of the water-producing zone, use of gravel packing or placing of cement plugs inside the casing, or the use of cement squeeze, or finally, the use of inorganic gels (such as sodium silicate) or organic polymeric gel systems.
The non-polymeric systems, e.g., cement squeeze, were used and reported by some researchers with some limited success. The use of inorganic gels, e.g., sodium silicate with hydrochloric acid, was tried by many researchers, also with some limited success. Organic polymer gel systems have also met with limited success.
Thus, a gelling agent for water shut-off in oil and gas wells solving the aforementioned problems is desired.